The present invention relates to electrical contact probes and, more particularly, to a barrel configuration for spring-loaded contact probes used in electrical testing applications such as providing electrical contact between diagnostic or testing equipment and an electrical device under test.
FIGS. 1 and 2 illustrate a conventionally configured prior art spring probe that is used to perform continuity and diagnostic electrical tests on an electrical device such as a printed circuit board (PCB) and the like. The prior art spring probe 10 comprises a barrel 12 configured in the shape of a cylindrical tube having a partially-closed terminal end 14 and an open plunger end 16. A plunger 18 is partially disposed within the barrel and comprises a first radially extending bearing section 20 at one end 22 of the plunger. The plunger also includes a head or tip portion 24 at an opposite end. The head portion is configured to accommodate electrical contact with a PCB under test. The plunger 18 further comprises an elongated second bearing section 26 that extends axially from the head portion 24 toward first bearing section 20. Between the first and second bearing sections is an elongated intermediate section 28 of the plunger. The sliding contact of the first and second bearing sections 20 and 26 against an inside diameter surface 30 of the barrel guides the plunger in its axial travel within the barrel upon contact of the plunger head portion with a PCB.
The plunger 18 is partially disposed within the barrel 12 such that the first bearing section 20 and a variable length of the second bearing section 26 are adjacent the inside diameter 30 of the barrel as the plunger reciprocates in and out of the barrel during use. The barrel includes a crimped portion 32 that extends a predetermined distance toward a center portion of the barrel and acts as a stop to retain the first bearing section 20 of the plunger 18 inside the barrel.
A coil spring 34 is disposed within the barrel 12 and is positioned between the partially-closed terminal end 14 of the barrel and the end portion 22 of the plunger. A ball 36 is disposed within the barrel between the partially-closed end 14 and an adjacent end portion of the spring 34. Upon contact by the head portion 24 against a PCB portion under test, the plunger slides axially into the barrel, causing plunger end portion 22 to travel toward the partially-closed terminal end 14 of the barrel and compresses the spring. The action of the spring resisting such compression imposes a simultaneous and opposite spring force on the PCB by the plunger head portion 24, thereby providing electrical contact under the mechanically applied spring contact between the plunger and the PCB.
Methods of contacting the spring probe to a sip pin or probe pad (not shown) include surface mounting techniques. In the surface mounting connection, the partially closed terminal end of the spring probe rests on the probe pad or sip pin in a noncompressed state of the probe, and in a compressed state of the probe the terminal end is compressed against the probe pad or sip pin to make an electrical connection.
A problem associated with the surface mounting spring probe arrangement is that the pressure per unit area of the terminal end of the barrel on the contact surface is low because the relatively large surface area of contact. This low force per unit area results in poor electrical contact between the barrel and the sip pin or probe pad particularly when surface imperfections on the sip pad or probe plate are present. In the prior art designs, terminal end of the spring probe is doughnut shaped. As pressure is equal to force over area, the amount of pressure is inversely proportional to the amount of area. The doughnut shape of the terminal end of the spring probe creates a relatively large contact surface area, and results in a relatively small contact pressure per unit area with the probe pad. Consequently a need exists for a terminal end design for the barrel which increases the force per unit area.
The present invention provides a newly designed barrel configuration, and particularly a crown shaped contact barrel configuration for a spring probe which increases the force per unit area by decreasing the unit area of contact between the terminal end of the spring probe and the probe pad or sip pin.
The tubular barrel has a hollow interior, a plunger end at one end of the barrel, and a terminal end at an opposite end of the barrel. The terminal end has a drawn opening with a circular cross-sectional dimension and a crown shaped contact integrally formed in the circular opening. There are several possible shapes for the crown shaped contact including preferably pointed projections where multiple triangular shapes are placed end to end around the circumference of the terminal end, and less preferably a corrugated shape with squares spaced apart around the circular edge, a wavy shape along the circular edge, or a rounded shape around the edge.